Journal of Advanced Ceramics 2012, 1(3): 232-240 DOI: 10.1007/s40145-012-0024-y ISSN 2226-4108 Research Article
Dielectric and electrical properties of Na2Pb2La2W2Ti4Ta4O30 electroceramics
P. R. DASa,*, S. BEHERAb, R. PADHEEa, P. NAYAKc, R.N.P. CHOUDHARYa
aDepartment of Physics, Institute of Technical Education & Research, Siksha ‘O’ Anusandhan University, Bhubaneswar-751030, Odisha, India; bDepartment of Physics, Hi-tech College Engineering, Bhubaneswar- 75102, Odisha, India; cSchool of Physics, Sambalpur University, Jyoti Vihar, Burla -768019, Odisha, India
Received August 23, 2012; Accepted October 6, 2012 © The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract: The polycrystalline sample of complex tungsten-bronze type compound (Na2Pb2La2W2Ti4Ta4O30 ) was prepared by a high-temperature solid-state reaction technique. Room temperature preliminary structural study using X-ray diffraction (XRD) data exhibits the formation of a single-phase new compound. The SEM micrograph of the compound exhibits non uniform rectangular grains distributed throughout the sample surface. Detailed studies of dielectric parameters (εr, tan δ) as a function of temperature and frequency, and P-E hysteresis (spontaneous polarization) confirmed the existence of ferroelectricity in the material. Complex impedance spectroscopy analysis, carried out as a function of frequency at different temperatures, established a correlation between the microstructure and electrical properties of the material. The electrical relaxation process occurring in the material is temperature dependent. The activation energy found from the Arrhenius plot that the conduction process in the material is of mixed type. The nature of frequency dependence of ac conductivity suggests that the material obeys Jonscher’s universal power law. Key words: electronic material; ferroelectricity; XRD; electrical conductivity
meta tantalate [6], there has been rapid progress in 1 Introduction search of new TB-type materials, required for various electronic devices such as multi-layer capacitors, Tungsten-bronze (TB) compounds belong to an transducers, actuators, random access memory, important class of dielectric materials, which display microwave dielectric resonators, phase shifters, interesting ferroelectric, pyroelectric, piezoelectric and pyroelectric detectors, etc because of their relatively nonlinear optic behaviors [1-3]. Since the discovery of high dielectric constant and low dielectric loss [7-11]. ferroelectricity in lead meta- niobate [4,5] and lead Detailed literature survey on TB structure ferroelectrics (single crystal, ceramics and thin films) reveals that a lot of work has already been done on ferroelectric * Corresponding author. ceramics of this family of different chemical formula. E-mail: [email protected] Recent studies on some complex compounds of the Journal of Advanced Ceramics 2012, 1(3): 232-240 233
family such as Na2Pb2Pr2W2Ti4Nb4O30 [12], agate mortar and pestle. Initial firing or calcination of Na2Pb2Nd2W2Ti4Ta4O30 [13], Na2Pb2Sm2W2Ti4Ta4O30 the mixed material was carried out in a high-purity [14], K2Ba2Nd2Ti4Nb4W2O30 [15], alumina crucible at 1100 ℃ (decided by repeated firing) Li2Pb2Pr2W2Ti4Nb4O30 [16], Na2Pb2R2W2Ti4V4O30 for 4 h in an air atmosphere. The process of grinding (R=Gd, Eu) [17], Li2Pb2La2W2Ti4Nb4O30 [18] and calcination was repeated several times to complete ceramics showed the presence of diffuse phase the reaction for the formation of the compound. To transition in these TB ferroelectrics. TB structural check the formation of the compound, X-ray family have a general formula diffraction data/pattern were recorded over a wide [(A1)2(A2)4(C)4][(B1)2(B2)8]O30, where the A site is range of Bragg’s angle ()(02 22 80) at a usually occupied by mono to trivalent cations and B +6 +4 +5 +5 +5 scanning rate of 3 deg/min (on calcined powder) at sites by W , Ti , Nb , Ta or V ions. This complex room temperature using an X-ray powder formula can be simplified to A6B10O30 [19,20] by diffractometer (Rigaku Miniflex, Japan) with Cukα suitably selected ions at appropriate sites. The radiation (λ=1.5405 Å). The fine and homogeneous distributions of metal cations in different interstices powder of the compound was mixed with polyvinyl can improve physical properties such as electro-optic, alcohol as binder, and compacted into cylindrical nonlinear optic, elasto-optic, ferroelectricity and pellets by applying an uni-axial pressure of 4.5× pyro-electricity in these materials. Further, structural 6 2 10 Nm using a hydraulic press. The pellets were flexibility and chemical versatility of the materials sintered at an optimized temperature of 1150 ℃ for 4 h. make them more suitable for device applications [21]. The sintered pellets were polished, coated with Some electrical properties of TB structure compounds high-quality silver paste, and dried at 160 ℃ for 10 h exist due to the contributions of various components and process such as intra-grain, inter-grain and in order to remove moisture (if any) and then cooled to electrode/interface process. In addition to the above, room temperature, before taking any electrical some complex TB compounds are more attractive measurements. The unpolished flat surface of the pellet because of their relatively low dielectric constant, high was gold-coated by a sputtering technique to record the pyroelectric coefficient/figure of merit and low loss, surface morphology by scanning electron microscope and hence they are useful for devices. Though a lot of (SEM, JEOL JSM-5800). The electrical measurements work has been done on TB compounds [22-27], not of the silvered- pellet was carried out using a computer-controlled Hioki 3532 LCR Hitester in the much work on dielectric and impedance properties 2 6 have been reported so far on complex TB structured frequency range of 10 -10 Hz at different compounds having all the valence elements (I-VI). In temperatures (29-500 ℃) with a laboratory-designed view of the above importance and unavailability of sample holder and vertical pit furnace. The polarization experimental data on the complex systems we have (hysteresis loop) of the material on the poled sample (6 synthesized and characterized such type of many kV/cm at 80 ℃ for 10 h) was obtained at room as well materials. In this paper we report structural, dielectric as other temperatures using P-E loop tracer (M/S and electrical properties of Na2Pb2La2W2Ti4Ta4O30 Marine India, New Delhi).
2 Experimental 3 Result and discussion
The polycrystalline sample of Na2Pb2La2W2Ti4Ta4O30 3.1 Structure/microstructure (abbreviated as NLaT) was prepared by a The XRD pattern (Fig. 1) of the compound shows high-temperature solid-state reaction technique using sharp and distinct peaks, which are different from that high purity (AR grade) raw materials: Na2CO3 (99%, of the ingredients, suggesting the formation of a new M/s s.d. Fine chem. Ltd.), PbO (99.9%, E. Merck Ltd., single phase compound. The diffraction peaks of the India), TiO2, WO3, Ta2O5 (99%, M/s Loba Chemie Pvt. compound were indexed in different crystal systems Ltd., India), and La2O3 (99.9%, Indian Rare Earth Ltd., and unit cell configurations using a standard computer India) in suitable stoichiometric proportion. The program package POWD [28]. An orthorhombic unit ingredients were thoroughly mixed and grinded in dry cell was finally selected on the basis of the best (air) and wet (methanol) medium for 1 h each in an agreement between observed (obs) and calculated (cal) 234 Journal of Advanced Ceramics 2012, 1(3): 232-240
2500 rod- and plate -like grains are uniformly distributed (with some voids) over the entire surface, density of 2000 the sample can be considered reasonably high for this type of materials. This conclusion is consistent with (331) 1500 (610) density determined by physical method (i.e., >90% of (87 1)
(131) the theoretical density). The average grain size is found (801) (370) (7 12 0) (7 1000 (12 6 0) (12 5 0) (11 1 0) (11 Intensity (a.u)
(4 0 2) (4 to be in the range 3-13 µm. (361) (10 0 2) (081) (071) (10 3 0) (14 0 0) (5 10 0) 10 (5 (6 14 0) (7 11 0) (542) (5 0 0) 0 (5 (62 1) (101) (341) 500 (670) 3.2 Dielectric study The temperature-frequency dependence of dielectric 0
20 30 40 50 60 70 80 constant and tangent loss is shown in Fig. 3. It is BraggBragg Angle angle (2 ) observed that both εr and tan δ decrease with increase Fig. 1 Indexed X-ray diffraction pattern of NLaT in frequency, which is a general feature of polar at room temperature. dielectrics [29]. As the compound has a dielectric anomaly at 342 K, existence of ferroelectric to inter-planar spacing d (i.e., ddd()obs cal paraelectric phase transition in the compound may be minimum). The lattice parameters of the selected unit considered. The broadened dielectric peak observed in cell were refined using the least-squares sub-routine of the compound suggests the phase transition of POWD, The refined lattice parameters are: a= diffused- type. It is also observed this transition 19.2384(14) Å, b=18.6641(14) Å, c=3.4619(14) Å, temperature (Tc) is frequency independent, and thus Volume=1243.0556 (Å)3 (estimated standard deviation can be classified as a non-relaxor ferroelectrics. The in parenthesis). The unit cell parameters and crystal maximum value of dielectric constant (εmax) at Tc for system of the compound are very much consistent with 10 kHz, 100 kHz and 1 MHz is 316, 196 and 143 those reported earlier [14]. The orthorhombic respectively, which are higher than those of reported distortion calculated as, =(ba)/(b+a), is found to be ones [13]. Similarly, an anomaly was observed in tanδ. 0.0151, well within the acceptable limits. The TB at 343 K, which is analogous with the anomaly structure is built on five crystallographic sites. It is observed in some TB type of compounds [30]. The difficult to precisely determine the R+3 ions increasing value of tan δ at high temperature region coordination (12- or 15- fold coordination) based on may be considered due to the existence of space charge the current results. However, the previous structural polarization and reduction of ferroelectric domain wall studies show that the rare earth cations predominantly contribution [14]. The maximum value of tan δ at Tc is prefer the A site [6,18]. found to be 2.25, 0.63 and 0.21 at 10, 100 kHz and Figure 2 shows the SEM micrograph of the sintered 1 MHz respectively. As the material has diffused phase pellet recorded at room temperature. The micrograph transition, it is required to estimate the degree of shows polycrystalline texture of the material. As the disorder. The calculated value of the degree of disorder or diffusivity (γ) using the empirical relation: 11 ln lnKg ln( TTc ) [31], with r max constant K, is found to be 1.79 at 10 kHz (Fig. 4), which supports higher degree of disordering in the system [32]. The diffused phase transition in TB compounds may be considered due to randomness of atoms distribution and in-homogeneity. It is known that TB structured compounds lose oxygen during high temperature sintering [33,34] which, follows the Kroger and Vink relation: Oo→1/2O2↑+Vo+2e , where Vo denotes oxygen vacancy [35]. Some oxygen vacancies created during high temperature processing Fig. 2 SEM micrograph of NLaT. induce disordering in the system [36]. As a result Journal of Advanced Ceramics 2012, 1(3): 232-240 235
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 3
0 2 30 C 2 r r tan tantan 300 r 2 10kHz 10kHz 2Pr=1.01 C/cm 100kHz 100kHz 2 Ec=0.1423kV/cm 1MHz 1MHz 1 1 ) 2
r r r
0 0 1 tan
200 tan C / cm C / P P ( -1 -1
0
100 -2 -2 300 400 500 600
Temperature(K) -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 E (kV / cm) Fig. 3 Variation of r and tan of NLaT as a E function of temperature at 10 kHz, 100 kHz and Fig. 5 Hysteresis loop of NLaT. 1 MHz respectively. used to analyze the electrical response (i.e., transport -2 properties) of polycrystalline materials (ferroelectric, ionic conductor, etc) in a wide range of frequencies.
-3 This technique separates the contributions of (i) bulk,
) (Experimental data) (ii) grain boundary and (iii) electrode polarization in max
) (Linear fit)
/ complex impedance and other related electrical max
1 -4 r -1/ r =1.79 parameters with different equivalent circuits. The / impedance parameters of the materials give us data ln(1/ ln(1
-5 having both real (resistive) and imaginary (reactive) components. The following basic equations of impedance, electrical modulus and related parameters -6 0.8 1.6 2.4 are normally used in this technique: complex ln(ln(TT -Tc)(KTc) (K) ) j impedance, ZZjZR() , complex Fig. 4 Variation of ln(1/εr–1/εmax) with ln(T–Tc) at s C 10 kHz of NLaT. s 1 modulus, M ()MjMjCoZ , complex diffuse type of ferroelectric to paraelectric phase () transition occurs. As the value of γ is between 1 and 2 admittance YY** jY jCR ()1 j C (γ=1 obeying the Curie Weiss law and γ=2 for oP p completely disordered system), confirms the and complex permittivity * j , where ω=2πf diffuse-phase transition in the compound. is the angular frequency; C0 is the geometrical 3.3 Polarization study capacitance, j 1 . The subscripts p and s refer to the equivalent parallel and series circuit components Figure 5 shows the polarization – electric field (PE) respectively. The complex impedance of a cell hysteresis loop of the compound. The remnant configuration of electrode/ceramic/electrode can be polarization (2Pr) and coercive field (Ec) of the explained in terms of the sum of the RC circuits in compound at 30 ℃ were found to be 1.01 C/cm2 and parallel combination mode. 0.1423 kV/cm, respectively. The appearance of Figure 6a shows the variation of Z' and Z'' with hysteresis loop confirms the ferroelectric properties at frequency at different temperatures. The value of Z' room temperature. decreases with the rise in both frequency and temperature which is related to the electrical 3.4 Electrical properties conductivity of the material. At higher frequency the 3.4.1 Impedance study value of Z' (for selected temperatures) coincides with The complex impedance spectroscopy (CIS) [37] is each other suggesting the release of space charges. 236 Journal of Advanced Ceramics 2012, 1(3): 232-240
strongly temperature dependent. The temperature (a) dependent slope clearly suggests that there are two 1000 ZZ'′ 1000 3500 C℃ distinct dispersion mechanism involved in the sample. 0 800 375 C℃ 800 The asymmetric nature of the plot [40,41] can be 4000 C℃ 0 explained using the equivalent circuits: CQR for low 425 C℃
)
) 600 600 ) temperature and (CQR) (CR) for high temperature (Fig. ) Z''Z″ (k
(k 0 ′ 350 ℃C ″ 7, inset). Jonscher’s universal capacitances can be
Z''(k Z Z Z'(k 400 0 400 375 ℃C expressed as, C=A(jω)m1 and Q=A(jω)n1 [42]. The 400 0℃C 0 200 425 ℃C 200 frequency dependence of the complex impedance can fit data R be expressed as Z*= o [41] where 0 0 mn jj 0.1 1 10 100 1000 1 12 FrequencyFrequency(kHz) (kHz) (a) Variation of Z′ and Z″ with frequency at selected ω1(=2f1) and ω2(=2f2) are the first and second temperatures characteristic angular frequencies respectively, exponent m is for high frequency and n is for low frequency. An excellent agreement between experimental and calculated values for both real and 0.75 imaginary parts of impedance is obtained from m nn non-linear fitting curve (Fig. 6a) using the formula
0.50 II Ro Z = mn [41]. & n
m m & n n m & 0.25 12 The variation of fitting parameters (m and n) with TTc c temperature is shown in Fig. 6b. It is seen that m is 0.00 close to unity and temperature independent. On the 0 100 200 300 400 o other hand the value of n is much less than one, and is TemperatureFrequency ( ℃( C)) temperature dependent. The value of n first decreases (b) variation of fitting parameters (m & n) with with rise in temperature and then attains minimum near temperature Tc. Again it increases slowly. The minimum value of n Fig. 6 near Tc can be explained by considering the restoring force between charge carriers and lattice [41]. The above variation in the value of n can be explained by The decrease in the value of Z' with temperature the theory given by Dissado and Hill [43,44]. suggest the NTCR behaviour of the material. The Z'' According to their theory, the exponent n characterizes attains a maximum value at a particular frequency, the magnitude of the correlation in a single dipole known as electrical relaxation frequency (ωmax) and reorientation. The unit value of n corresponds to fully then decreases. The appearance of peaks in the loss correlated transitions, and zero corresponds to fully spectrum exhibits the existence of relaxation properties uncorrelated transition. In our experiment n tends to a in the material. The broadening of peaks on increasing minimum value near Tc suggesting a strongly temperature confirms the existence of (i) temperature uncorrelated reorientation of the charge carrier dependence of relaxation process and (ii) diffused polarization at transition points. These results and phase transition phenomena in the material [38]. The conclusion are very much consistent with those relaxation species may be electrons in low temperature reported earlier [30]. region whereas defects/vacancies in higher temperature Figures 7a, 7b show the Nyquist- plots (Z' versus region may be responsible for electrical conduction in Z'') with experimental and fitted data using the material [39]. In the imaginary plot of the ZSWIMPWIN ver. 2 [45] of NLaT at selected impedance, the high frequency slope is independent of temperatures. This plot has one semicircular arc in the temperature. On the other hand, low frequency slope is low temperature region (200-350 ℃) whereas at higher Journal of Advanced Ceramics 2012, 1(3): 232-240 237
1200 hence justify the presence of constant phase element 10000 0 (a) 350200C0C (b) 0 Cb 200 C fit Cgb (CPE). This behavior follows the Jonscher’s universal 80 00 0 350 225Cfit0C 2250 0Cf it 375 C0 60 00 250 C power law. Therefore a constant phase element (CPE) 0 0 ) 375 250CfitCf it CPE 0 40 00 can be introduced in the equivalent circuit. It shows the 800 Z''(k 400 C 400 0Cfit 20 00 Rb 0 Rgb power law dependence of the impedance over several
) 425 C ) 0 C Cfit C Cfit C Cfit C Cfit 0 425 Cfit 0 0 0 0 0 0 0 0 decades of frequency domain [50]. As the temperature 0 200040006000800010000 (k
″ Z' (k) Z''(k 350 350 375 375 400 400 425 425
Z increases, the intercept of the semicircles at the Z'-axis 400 shifts towards lower Z' values suggesting the reduction of the grain (bulk) resistance. This confirms the negative temperature co-efficient of resistance (NTCR) 0 0 400 800 1200 behavior of the material. Further, the depressed ZZ′'(k(k)) semicircles have their centers below the real axis, (a) which again indicates the departure from the ideal 10000 Debye-type behavior [37]. Above observations suggest 0 (b) 200 C that there is a distribution of relaxation time instead of 2000C fit 0 a single relaxation time in the material [30]. 225 C 2250Cfit The variation of relaxation time (τ) with reciprocal 0 3 250 C of temperature (10 /T) is shown in Fig. 8. This graph 0
250 Cfit ) follows the Arrhenius relation, τ=τoexp(Ea /KBT) 5000 (k
″ where the symbols have their usual meanings. The Z relaxation time (τ) was calculated from Z'' versus frequency plot using the relation τ=1/ω=1/2πfmax, where f is the relaxation frequency. It is observed max that the value of τ decreases with an increase in temperature like- semiconductor. The value of 0 activation energy (Ea) was found to be 0.92 eV which 0 5000 10000 Z′(k) agrees well with that of a semiconductor. 3.4.2 dc conductivity Fig. 7 Variation of Z″ with Z′ at different Figure 9 shows the variation of dc conductivity with temperatures. respect to inverse of absolute temperature. The value temperatures (375-425 ℃) two semicircular arcs (with of bulk conductivity of the material was evaluated centre below the real axis) are observed. This indicates from the complex impedance plots of the sample at that transport properties of the material are mainly due different temperatures using the relation: dc=t/Rb A, to the bulk (intragrain) at low temperature, and due to where Rb is the bulk resistance, t the thickness and A is bulk and grain boundary at higher temperatures [46]. the surface area of the sample. It is found that the dc Most widely accepted approach to interpret the -2 depression of semicircles is statistical distribution of Experimental Data Linear fit relaxation time (non-Debye type of relaxation) in the material [47]. The equivalent circuit consists of parallel combination of (CQR) and (CR), where Q is known as -4 constant phase element (CPE). The admittance (Y) of (sec) (s) n n n CPE is expressed as: Y (CPE)=A0(j) =A +jB , log log where A=A0cos(n/2) and B=A0sin(n/2). The values of A0 and n are frequency independent but temperature -6 dependent. A0 is the magnitude of the dispersion, and 01.n For an ideal capacitor n=1 and for ideal 1. 6 2.4 3.2 resistor n0 [48]. Further, the non-coincidence of Z'' 101033//T(KT(K-1)1) spectra with frequency in all the temperature range Fig. 8 Variation of relaxation time (τ) as a function confirms the departure from ideal Debye type [49], and of reciprocal of absolute temperature. 238 Journal of Advanced Ceramics 2012, 1(3): 232-240
the thermally activated transport properties of the E a KBT material obeying Arrhenius equation: ac o e , where the symbols have their usual meanings. The
) 1
) anomaly in slope of ac conductivity of the sample at -1 1E-7 342 K corresponds to the anomaly observed in our
(mho·cm dielectric study. The ac conductivity pattern indicates a (mho.cm dc
dc
progressive rise with temperature having maximum
enhancement in at higher frequencies. The Ea of the sample is found to be 0.65 eV in the ferroelectric phase
1E-8 (
1.6 2.0 10 kHz. Similarly, for 100 kHz Ea is 0.44eV (
σac=ωεrεotanδ, where ε0 is the vacuum dielectric increases with increase in temperature and obeys the constant and ω is the angular frequency. The nature of phenomenological law (Jonscher’s universal power n variation of σac over a wide temperature range supports law): σac=σdc+Aω [38], where dc is the frequency
1E-5 10kHz 100kHz
line shows ac conductivit y fitting 1E-6
) 1 1E-4 ) ) ) 1 -1 -1 ·m 1E-7 1 m .m ·m -1 -1 1 ( ( ( ac ac ac ( ac 0 100 C 1E-8 1250C 1500C 1E-5 1750C
1E-9 1.6 2.4 3.2 0. 1 1 10 100 1000 33 -11 Frequency (kHz) 1010 //T(KT(K ) ) Frequency(kHz) Fig. 10 Variation of ac conductivity with inverse Fig. 11 Variation of ac conductivity with frequency of absolute temperature at different frequencies. at different temperatures. Journal of Advanced Ceramics 2012, 1(3): 232-240 239
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